In PEM fuel cell systems, it is well known that, when the electrical circuit is opened and there is no longer a load across the cell, such as upon and during shutdown of the cell, the presence of air on the cathode, coupled with hydrogen fuel remaining on the anode, often cause unacceptable electrode potentials, resulting in catalyst and catalyst support oxidation and corrosion and attendant cell performance degradation. Inert gas has been used to purge both the anode flow field and the cathode flow field immediately upon cell shutdown to passivate the anode and cathode so as to minimize or prevent such cell performance degradation.
It is desired to avoid the costs associated with storing and delivering a separate supply of inert gas to fuel cells, especially in automotive applications where compactness and low cost are critical, and where the system must be shut down and started frequently. In U.S. Pat. No. 6,635,370, a fuel cell system is shut down by disconnecting the primary load, shutting off the air flow, closing air inlet and air outlet valves and controlling the fuel flow into and out of the system in a manner that results in the fuel cell gases coming to equilibrium across the cells, with the fuel flow shut off, with gas composition of a small amount of hydrogen, balance fuel cell inert gases. These inert gases do not react with hydrogen or oxygen within the fuel cell, and do not otherwise harm cell performance to any significant extent, and are, therefore, harmless to the fuel cell. Fuel cell inert gases may also include trace amounts of elements found in atmospheric air. If the fuel is high purity hydrogen and the oxidant is air, the “balance” fuel cell inert gas will be substantially all nitrogen, with a small amount of carbon dioxide found in atmospheric air, plus trace amounts of other elements found in atmospheric air.
In the aforementioned patent, after disconnecting the primary load and shutting off the air supply to and exhaust from the cathode flow fields, fuel continues to be fed to the anode flow fields until the remaining oxidant is consumed. This oxidant consumption is aided by recycling gas from the cathode exit to the cathode inlet, and by having a small auxiliary load applied across the cell, which also quickly drives down the cathode potential. Recycling the cathode gas assures good mixing of the remaining gas in the cathode, so that oxygen will be spread more uniformly throughout the fuel cells and thereby be more quickly consumed.
As the cathode gas is recycled, hydrogen in the anode flow field diffuses to the cathode through the membrane so that the oxygen in the cathode flow field is consumed, resulting in a total lesser volume of gas in the cathode flow fields, with an increasing concentration of nitrogen and other gases found in the atmosphere. The consumption of oxygen from the cathode flow fields results in a gas pressure drop in the cathode. When the cathode inlet and exit valves are closed, a vacuum is formed. Any water remaining in the coolant flow channels adjacent to the porous, hydrophilic oxidant reactant gas flow field plates, with no positive pressure differential between the cathode flow fields and the coolant channels, will flow into the cathode flow fields. This is sometimes referred to as water “slump”.
Coolant plates that are both porous and hydrophilic are sometimes called water transport plates (WTPs). The WTP allows coolant from the coolant channels to flow both through the plane and in the plane within the plate. The WTP is distinguished from fuel cells with solid cooler plates by having a direct interface between the reactant gases and coolant. As a result, there is a criticality to balancing the pressure between the reactants and the coolant in order to maintain the location of coolant and reactants within the coolant section of the cell structure. Without a positive reactant gas pressure over coolant pressure, the coolant stream could flood the reactant cavities with coolant as claimed in U.S. Pat. Nos. 5,705,951 and 5,853,909. A fuel cell shut down with coolant/water contained in the reactant cavities will be more difficult to start and may be impossible to start from a frozen condition.